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Back Plates.

345

by milling or chipping. Mark the flange rivets, one in six, with a
special steel strip, and the rivets along the seam a d, one in six,
from the tube plate. Set out the centres for stays Y YI} Fig. 311,
and niudholes z z, as shewn at 14. Next prepare the stiffening
plates 15 by marking out, sawing, cutting the oval hole by the
special method shewn in Fig. 291, and drilling the rivet holes.
Place the stiffening plates in position, and mark through all their
holes; then drill all holes by a Radial Machine, and cut the
mudholes by the appliance in Fig. 291. The edges of the furnace
flanges are tooled in the same machine by fixing the plate hori-
zontally on the table and revolving the tool Q Q, as at 16.

IV.   Top Back Plate is prepared in the same manner as I.

V.  Back Middle Plate.—This must be lined out as at  17,
with a and b parallel, and the curves struck.    The rest may be
understood from II.    After planing a and £, and setting out the
stay holes, the latter are left to be drilled till all are bolted
together.

VI.  Bottom Back Plate (18) is treated in the same manner
as I., but the stay holes are all drilled in position, as in last
example.

VII.  Front Ring Plates.—There are three of these, all equal
in length.    They are lined, as at 19, with long set squares, then
planed, the long edges to a bevel, and the short edges square;
next taken to the Rolls, Figs. 286-7, an(i Put through in the
manner previously described.    But many Marine firms prefer to
work with Vertical Rolls, believing that besides supporting the
weight, the curve is obtained more squarely with the long edge.
In finishing the short edge, a greater pressure is given to secure
accuracy of curvature, and partially avoid the necessity of bending
with hammer.   Now mark off the rivet holes to suit those already
drilled in the flanges of the Front and Back plates.    To this end
the steel strips are again used, and, being very thin, do not differ
appreciably in their outside and inside circumferences.    The
positions of joints T x must be found with relation to the butt
joints F F (Fig. 311), and the centres of XT marked upon the
front long edge of the ring plates.    Then the steel strips are
applied^ and the holes marked to correspond with the flanges.
Of course these strips must be all carefully numbered, to avoid